Entry of Salmonella into host enterocytes relies on its pathogenicity island 1 effector SipA. We found that SipA binds to F-actin in a 1:2 stoichiometry with sub-nanomolar affinity. A cryo-EM reconstruction revealed that SipA’s globular core binds at the groove between actin strands, whereas the extended C-terminal arm penetrates deeply into the inter-strand space, stabilizing F-actin from within. The unusually strong binding of SipA is achieved by a combination of fast association via the core and very slow dissociation dictated by the arm. Similar to P_i, BeF₃, and phalloidin, SipA potently inhibited actin depolymerization by actin depolymerizing factor (ADF)/cofilin, which correlated with increased filament stiffness, supporting the hypothesis that F-actin’s mechanical properties contribute to the recognition of its nucleotide state by protein partners. The remarkably strong binding to F-actin maximizes the toxin’s effects at the injection site while minimizing global influence on the cytoskeleton and preventing pathogen detection by the host cell.

沙门氏菌进入宿主肠细胞依赖于其致病性岛1效应子SipA。我们发现 SipA 以 1:2 的化学计量比和亚纳摩尔亲和力与 F-肌动蛋白结合。冷冻电镜重建显示，SipA 的球状核心结合在肌动蛋白链之间的凹槽处，而延伸的 C 端臂则深入到链间空间，从内部稳定 F-肌动蛋白。 SipA 异常强的结合是通过核心的快速结合和臂控制的非常缓慢的解离的结合来实现的。与 P、BeF ₃ 和鬼笔环肽类似，SipA 通过肌动蛋白解聚因子 (ADF)/cofilin 有效抑制肌动蛋白解聚，这与肌丝刚度的增加相关，支持 F-肌动蛋白的机械性能有助于肌动蛋白丝刚性的假设。蛋白质伙伴对其核苷酸状态的识别。与 F-肌动蛋白的极强结合使毒素在注射部位的作用最大化，同时最大限度地减少对细胞骨架的整体影响并防止宿主细胞检测到病原体。